Field of the Invention
This invention relates to molding and extrusion of plastic resin pellets to produce finished and semi-finished plastic products, and more particularly relates to using liquid color to color those finished and semi-finished plastic parts as the parts are being molded or extruded.
Description of the Prior Art
In the plastics industry, injection molding machines and extruders (herein collectively referred to as “process machines”) include a screw inside a barrel for melting the plastic resin pellets by working it by action of the screw against the resin as the screw rotates, driving the melted plastic forward through the barrel. The plastic resin pellets are fed into the barrel at one end of the barrel. At this end, referred to as the “feed end”, there is an opening in the barrel, above the screw, through which the plastic resin pellets fall by gravity into the barrel adjacent to and in contact with the screw. This opening is referred to as the “feed throat” of the process machine.
A hopper or other plastic resin pellet delivery apparatus is positioned above the feed throat. When liquid color is used to color the finished or semi-finished plastic products produced by molding or extrusion, the goal is to deliver the liquid color directly into the area of the screw in such a way that the liquid color will not come in contact with other surfaces before contacting the screw. Problems can arise if the liquid color contacts any portion of the feed throat as the liquid color is being fed to the screw. Liquid color for coloring plastics is extremely viscous, somewhat like tar, and hence is very difficult to handle!
When liquid color is used to color a molded or extruded product, the liquid color is delivered directly into the plastic molding or extrusion process. The goal is to deliver the liquid color into the screw area in such a way that liquid color will not come into contact with other surfaces before entering the barrel and effectively being drawn into the resin by the rotating action of the screw within the barrel. If the liquid color touches even the sides of the feed throat, this can present a problem due to the difficulty of clean up and the possibility of contamination of one color with another when color changeover is effectuated.
In the prior art, the most accepted way to introduce liquid color into the screw barrel and adjacent to the rotatable screw is to introduce the liquid color through a guide tube that passes through the feed throat and has its outlet end positioned close to the screw, so the liquid color does not exit the guide tube until it is well past the surfaces of the feed throat and other components, where spillage of the liquid color on to those surfaces could cause a problem. Contact of liquid color on those surfaces must be avoided to avoid the difficulties of clean-up and the problems of cross-color contamination.
Currently, the most widely used technique introduces the liquid color to the screw through a tube that is inserted into the feed throat, with the outlet of the tube positioned very close to the screw. With the tube outlet being positioned very close to the screw, liquid color does not exit the tube until the liquid color is well past all surfaces of the feed throat and other surfaces where a problem might arise if those surfaces were contacted by the liquid color.
In current practice this is accomplished by first installing an adaptor plate, which is typically about ¾ of an inch thick, over the opening to the feed throat. The opening to the feed throat is underneath a hopper or other device supplying plastic resin pellets to the process machine. The adaptor plate is preferably sandwiched between the hopper and the opening to the feed throat. The adaptor plate typically has a slot milled into it, allowing a short length of rigid copper tubing to be permanently positioned in the slot. The rigid tubing is typically ⅝ inch outer diameter, ½ inch inner diameter copper tubing. This rigid copper tubing provides a controlled fixed path leading from outside the process machine into and through the feed throat to a point just above the screw. This permits a smaller color delivery tube to be inserted into the rigid copper tube. The larger, rigid tube is formed in such a way as to direct the smaller color delivery tube downwardly to a desired point very close to the surface of the screw at the bottom of the feed throat below the adaptor plate.
The smaller, color delivery tube is flexible and is inserted into the rigid tube and removed from the rigid tube as required.
In the prior art, changing colors to produce parts of different color requires that the color delivery tube be removed and another color delivery tube, for use in supplying a new color, be inserted into the rigid tube. When there is to be a changeover in color for parts being fabricated by a given process machine, the new color must be delivered using a new, clean color delivery tube. or a color delivery tube that has previously been used to deliver that very same color. In this way, the color delivery tubes do not need to be cleaned, and there is no danger of cross-contamination of colors. Each color has its own dedicated color delivery tube.
A problem with current liquid color delivery systems as described immediately above is that when a color delivery tube is withdrawn, liquid color will drip from the open outlet end of the color delivery tube. Any dripping of the liquid color creates a mess that is difficult to clean up due to the high viscosity, staining and other properties of liquid color. Typically, the mess only gets worse and worse as time passes using current liquid color delivery techniques.